This invention relates to coating both major surfaces of ceramic substrates which, when coated, are especially useful as microcircuits, integrated circuits, and other electrical, electroconductive, magnetic, and like components. Circuits and circuit components of the types mentioned are often fabricated by thin film application techniques. Recently, the materials used in such film circuit elements have included a wide variety of conductive, resistive, dielectric, semi-conductive, superconductive and magnetic films, as well as various substrates. The most commonly used conductive films are metals such as copper, aluminum, gold, platinum, and tantalum. Electrically resisting films include anodized tantalum, tin oxide, indium oxide, and other. Useful dielectric films include the oxides of silicon and tantalum. Semi-conductive materials such as silicon, germanium, and compound semi-conductors have been used. Tin and its alloys are under study as superconductive films. Permalloys and various special alloys are included among magnetic films.
The substrates should have properties which are compatible with the thin film materials being applied. The substrate should be capable of tolerating the process conditions required for the deposition of films, should have a high electrical resistivity, a low dielectric loss and good chemical durability. The substrate should be free of impurities which adversely affect circuit performance. The smoothness of the substrate surface is usually important. Surface irregularities must be avoided for certain applications.
A wide variety of substrate materials have been studied including glasses, ceramics, glazed ceramics, crystals, dielectric coated metals, glazed ceramics, and organic polymers.
Ceramics have special characteristics which make them an important substrate material, particularly, their high softening temperature and their low electroconductivity. However, the surface irregularities of ceramics in deviations from flatness is usually greater than that of glass. Glass and ceramics have been glazed previously on a single surface. In U.S. Pat. No. 3,223,549 to Fredley et al., for example, glass substrates are supported on their under surfaces on a gas bed to minimize thermal deformation while a coating is developed on the upward facing major surface of the glass substrate during an elevated temperature cycle. The gas bed maintains a major surface of the glass substrate out of contact with solid means that would introduce surface irregularities in the substrate on contact with said surface, particularly at the higher temperature regions of the cycle.
As the art required higher component density within a given space, it has become advantageous to glaze both sides of each substrate, thus doubling the useable area. However, applying a glaze to both sides of the substrate introduced a problem of supporting the substrate during firing of the coating material. In order to coat both surfaces, the substrate was fired while supported vertically on two parallel edges from which the coating material had been removed. Some warp resulted from the manner in which the substrate was supported in firing.
U.S. Pat. No. 3,558,346 to Plumley minimizes this warpage by supporting a substrate on one side while applying a ceramic frit to the other side. The substrate warpage is minimized because the coatings are applied to the opposite surfaces in consecutive steps. The substrate is supported on a planar refractory support during each of the two firing operations.
In accordance with the Plumley patented invention, the two glazes can be consecutively applied only if the maturing temperature of the second glaze is below the temperature at which the first glaze becomes deformed or marred. As used herein, the term "maturing temperature" refers to a temperature at which a glass frit must be held for a selected time to cause the frit particles to consolidate and form a smooth, continuous, adherent, glassy coating. Since frit compositions having lower maturing temperatures are generally less durable than frit compositions with higher maturing temperatures, the invention suggested in the Plumley patent resulted in articles having a coating of relatively low durability on one side. Furthermore, it is necessary to heat a glass substrate to a higher temperature for tempering than is needed to mature a frit, particularly one having a low maturing temperature. Therefore, the method of the Plumley patent would not be suitable to produce a tempered, frit-coated glass article.